Butterflies flip switch to look deadly

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Great pretender Scientists have discovered how one species of butterfly mimics other species to avoid being eaten by birds, solving a mystery that has puzzled scientists since Darwin.

The research, which was published today in Nature, looked at the Amazonian butterfly Heliconius numata. Members of a single population may exhibit up to seven different wing patterns, with each type looking exactly like those of unpalatable butterfly species belonging to the distantly related Melinaea family.

"Charles Darwin was puzzled by how butterflies evolved such similar patterns of warning colouration," says Dr Siu Fai (Ronald) Lee from the Department of Genetics and Bio21 Institute at the University of Melbourne. He was part of the international research team, which was led by scientists at CNRS and the University of Exeter.

H. numata and other Heliconius species employ a strategy known as Müllerian mimicry, where two or more distasteful species with similar predators share the same warning marks.

Predators learn from unpleasant past experiences and avoid eating any similar looking animals in the future. Müllerian mimicry differs from Batesian mimicry, where a harmless species mimics the colouration of a poisonous one.

"The strategy, where two distant relatives converge into a single form means they share the cost of educating predators, in this case birds," explains Lee. "Some individuals are sacrificed in the process, but both species benefit."

Despite having the same DNA, H. numata butterflies can look completely different. In what is widely known as selection pressure, any that don't conform to the optimal colour pattern in that locality are likely to be eaten, and are unlikely to pass on their genes to the next generation.

Supergene flicks the switch

The scientists wanted to determine how the successful colour combinations arose in the first place. They also examined whether the mutation that results in one species adopting the warning colouration of another species occurs in a single lucky step, or is the result of incremental change in the population.

By mapping links between genetic markers in the offspring of adults with different wing patterns they found that the mechanism responsible for producing variation is contained in a single region of the chromosome.

Known as a supergene, this part of the chromosome uses a process similar to shuffling a small pack of cards.

"This supergene is a kind of switch that holds several mimicry factors together," says Lee.

"DNA along this chromosome flips to produce different combinations of the supergene, allowing the butterfly to morph into different forms," he says, adding that different types of supergene correspond perfectly to the most successfully adapted wing patterns found in this species.

"It allows mimicry factors that work best together to stay together, while preventing combinations that produce other patterns from continuing in the population," he says.

Adaptive significance

Supergenes are hotspots of adaption in other plant and animal species including the shape of primrose flowers and the pattern of snail shells.

This discovery also shows that small chromosomal changes can preserve successful gene combinations and help the species to adapt. The researchers say that unravelling the genetic basis of this, and other supergenes, will be key to understanding the mechanisms of adaptation in response to ecological change.

"This is a fascinating adaptive strategy and the genetic solution is surprisingly elegant," says Lee. "It keeps favourable factors together while stopping unfavourable combinations from arising, because any offspring born with the wrong colouration are quickly picked off by birds, removing themselves from the gene pool in the process.